1. Field of the Invention
The present invention relates to an electrophotographic image forming device such as a copier, a printer, or a facsimile machine, specifically, an image forming device able to reduce impact when a front end of a recording sheet runs into a space between an image carrier, an image transfer unit and a fusing unit, or when a back end of the recording sheet passes through the nipping portion.
2. Description of the Related Art
In an electrophotographic image forming device of the related art, like a copier, a printer, or a facsimile machine, a toner image is carried on a photoconductive drum or a transfer belt while being conveyed, and a transfer roller presses a recording sheet, like recording paper, against the transfer belt to transfer the toner image to the recording sheet. When a front end of the recording sheet runs into a nipping portion between the transfer belt and the transfer roller, the front end of the recording sheet collides with the transfer belt, and this may cause a temporary increase or decrease of the conveyance speed of the transfer belt.
In recent years, image forming devices having a transfer and fusing unit have been developed. For example, in one kind of such image forming device, a pre-heating unit heats the recording sheet immediately before the transfer roller presses the recording sheet against the transfer belt, thereby, transfer and fusing are carried out at the same time and at the same position; in an additional kind of image forming device, another intermediate image carrier at the next stage presses the recording sheet against the transfer belt to touch the toner image, and an over-heating unit is provided to heat the toner image in advance at a position before transfer so as to melt the toner beforehand and fuse the toner image at the transfer position.
In such kinds of devices, similarly, at the nipping portion, the front end of the recording sheet may collide with the transfer and fusing unit, and this may cause a temporary increase or decrease of the conveyance speed of the transfer and fusing unit. This phenomenon is noticeable when the recording sheet is thick.
In an image transfer device or an image transfer and fusing device that transfers the toner image on the transfer belt or the intermediate image carrier to the recording sheet, the temporary increase or decrease of the conveyance speed influences cleaning, exposure, and developing processes when these processes are performed on the surface of the same transfer belt. This causes image defects, also known as banding, like lines in the main scanning direction, and stripe-like dark and light color unevenness in the sub scanning direction.
To solve this problem, in an image forming device of the related art, according to the thickness of the recording sheet, a specified gap smaller than the recording sheet thickness is formed in the nipping portion between the transfer belt and the transfer roller.
For example, Japanese Laid Open Patent Application No. 61-90167 (hereinafter, referred to as “reference 1”) discloses an image forming device in which a gap is formed between a photoconductive drum and a pressure roller to reduce impact when the recording sheet runs into the space between the photoconductive drum and the pressure roller.
In addition, Japanese Laid Open Utility Model Application No. 56-47639 (hereinafter, referred to as “reference 2”) discloses an image forming device in which a depressed groove is formed between a driving roller and a driven roller to reduce impact when the recording sheet runs into the space between the driving roller and the driven roller.
In addition, Japanese Laid Open Patent Application No. 6-274051 (hereinafter, referred to as “reference 3”) discloses an image forming device in which rotation unevenness of a dielectric drum carrying an image is reduced by enlarging a gap between the dielectric drum and a pressure roller when a recording sheet runs into the gap; on the other hand, when an image is not formed on the dielectric drum, in synchronization with the timings when the recording sheet runs into a nipping portion between the dielectric drum and the pressure roller, the dielectric drum and the pressure roller are pressed to contact each other to close the gap in the nipping portion by a driving force from a driving unit for driving the dielectric drum.
In addition, Japanese Laid Open Patent Application No. 4-242276 (hereinafter, referred to as “reference 4”) discloses an image forming device in which there are provided a dielectric drum, a pressure roller, a swinging arm which supports the pressure roller, an eccentric cam which drives the swinging arm up and down, and a motor which rotates the eccentric cam, and due to a downward action of the swing arm caused when the motor rotates the eccentric cam, a gap of a nipping portion between the dielectric drum and the pressure roller is enlarged according to the thickness of the recording sheet being conveyed, thereby adjusting the level of push-down of the pressure roller.
However, in the image forming device of the related art, because the swinging arm which supports the pressure roller, the eccentric cam, and the motor are provided, and the amount of push-down of the pressure roller is adjusted to form a gap between the dielectric drum and the pressure roller, the mechanism for forming the gap between the dielectric drum and the pressure roller in the nipping portion is complicated, and a large number of parts is required.